<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>边框</title>
    <style>
        .c3{
            width: 0px;
            height: 200px;
            border-bottom: 30px solid red;
            border-left: 30px solid transparent;
            border-right: 30px solid transparent;
        }
    </style>
</head>
<body>
    <div class="c3"></div>
</body>
</html>
